“There are so many different agencies working together that we need to come together and develop a timeline,” she added.

A representative for the National Border Patrol Council (NBPC) labour union told KTSM-News: “We strongly believe rocks were what was used.”

Chris Cabrera, a spokesman for the union, told NBC News that border agents are often attacked with rocks.

“We’ve had agents, a good friend of mine actually almost lost his eye,” he said. “It happens quite a bit, unfortunately.”

He added that some of the projectiles hurled at officers are as big as a softball.

Art Del Cueto, the vice-president of the NBPC, said the agents “were out there responding to a sensor and one agent was murdered”.

The NBPC – which says it represents 75% of the 18,000-officer agency – has been a strong proponent of President Trump’s border wall.

“You hear the talk about individuals trying to… enter this country to get a better life but the reality is they will stop at nothing including killing a federal agent – including taking someone’s life which is what happened,” Mr Del Cueto added.

Border patrol records show that the Big Bend region, where the apparent attack took place, only accounts for 1% of apprehended migrants.

The region’s mountains, and the Rio Grande River, make it one of the more difficult areas to cross the border illegally, CBS News reports.